Vapor lamps, including fluorescent, mercury vapor, and neon lamps, all possess negative resistance characteristics. That is, the resistance of the lamp decreases with an increase in current. Therefore, without some sort of current-limiting device, after the lamp is initially started the current would rise swiftly until the lamp failed. These current-limiting devices, generally called ballasts, can be resistive, capacitive or inductive. Resistance ballasting is inefficient so that inductive and capacitive ballasts are much more common.
One type of fluorescent lamp uses what is called an instant-start circuit. This arrangement requires a relatively high voltage to the lamp so that the ballast is often a transformer with capacitors. When the lamp is operating normally, the ballast will exhibit a mid-range voltage, such as 350-400 volts. However, if the lamp is burned out, the ballast will exhibit a high voltage such as 750-900 volts. Most fluorescent light fixtures have a switch which disconnects the ballast from the terminals of these fixtures when the lamp is removed for safety during maintenance operation. Therefore, when the lamp is removed the voltage applied to the terminals of the fluorescent lamp fixture is zero.
One type of fluorescent lamp produces ultraviolet light and is commonly used in sterilization chambers. Since large doses of ultraviolet light is harmful to the human eye, the sterilizing chambers are typically enclosed. Such chambers often have a bank of ultraviolet lights, for example 12-20, within the sterilization chamber. One problem which arises is that if one or more of the lights burns out, the operator has no ready indication that such has happened. The result can be incomplete sterilization and subsequent growth of pathogens.
If desired, the operator of the sterilization line could monitor the light output from each individual lamp within a chamber via a separate monitor coupled to each individual lamp. However, such multiplistic monitoring can be expensive and may require more of the operator's attention than is desired. Therefore, what is missing in the prior art is an inexpensive device that would indicate to the operator when one or more of the ultraviolet fluorescent lamps are burned out.